The BGA (Ball Grid Array) semiconductor device is a type of semiconductor device having a surface assembly type package structure, in which plural ball-shaped solder pieces known as solder bumps are arranged two-dimensionally as the external connecting terminals. Since the connecting terminals can be arranged two-dimensionally, this type of package is preferred for high-density assembly. In particular, BGA has a very important position for the realization of CSP (Chip Size Package) semiconductor devices, that is, semiconductor devices having a package size that is nearly the same size or only slightly larger than the chip size, which have attracted much attention in recent years.
There are various types of BGA, such as the type in which a flexible insulating substrate made of polyimide resin or the like is used as the substrate of the package, and solder bumps are arranged two-dimensionally on one side of the flexible insulating substrate. In this type of semiconductor device, the semiconductor chip is attached to the flexible insulating substrate by an adhesive made of a nonconductive epoxy resin or the like and known as die attaching material. In the manufacturing process, an appropriate number of drops of the die attaching material is applied to the flexible insulating substrate; then, the semiconductor chip is pressed on the substrate, and the die attaching material spreads over the lower surface of the chip. After curing of the die attaching material by heating, necessary wiring is arranged and the semiconductor chip is sealed by a resin. For such a BGA type semiconductor device, assembly is carried out by melting the solder bumps as a whole by reflow after the semiconductor device is mounted on the external substrate (printed board for wiring) by a mounter.
High reliability during the assembly of the semiconductor device is very important. In particular, it has been noted that the assembly reliability is lower than that of the conventional QFP (Quad Flat Package) as explained below. In the temperature cycle test performed for the aforementioned BGA type semiconductor device, cracks may be generated in the joint between the flexible insulating substrate and the solder bumps, in particular, in the joint of the solder bumps right below the outer edge of the chip, and these can lead to the problem of openings in the device. The main cause is due to the difference in the linear expansion coefficient between the semiconductor chip and the external substrate, which generates a shear stress that is concentrated in the aforementioned joint. That is, since the modulus of elasticity of the die attaching material and the flexible insulating substrate between the chip and the external substrate is much lower than that of the chip and the external substrate, the shear stress generated due to the aforementioned difference in the linear expansion coefficient is concentrated in the solder joint. Since the influence of expansion of the semiconductor chip caused by heat is maximum at the edge, the shear stress is maximum in the joint of the solder bumps located right below it. Consequently, the stress in this portion rises, and the assembly reliability of the package is significantly affected.
The purpose of the present invention is to reduce the stress concentration in the joint between the flexible insulating substrate and the solder bumps located right below the outer edge of the semiconductor chip, to avoid the problem of openings (wire breakage) caused by cracks or the like in the joint, and thus to increase the assembly reliability.
Another purpose of the present invention is to improve the assembly reliability of the package without changing the basic shape of the aforementioned package, without a significant change in the manufacturing process and without increasing the manufacturing cost.